This invention relates generally to studying the natural heart in operation and, more specifically, to a system for doing so with the heart explanted outside of the body.
The natural human heart and accompanying circulatory system are critical components of the human body and systematically provide the needed nutrients and oxygen for the body. As such, the proper operation of the circulatory system, and particularly, the proper operation of the heart, are critical in the overall health and well-being of a person. A physical ailment or condition which compromises the normal and healthy operation of the heart can therefore be particularly critical and may result in a condition which must be medically remedied.
Specifically, the natural heart, or rather the cardiac tissue of the heart, can fail for various reasons to a point where the heart can no longer provide sufficient circulation of blood for the body so that life can be maintained. To address the problem of a failing natural heart, solutions are offered to provide ways in which circulation of blood might be maintained and improved.
Some solutions involve replacing the heart. Other solutions are directed to maintaining operation of the existing heart. One such replacement solution has been to replace the existing natural heart in a patient with an artificial heart or a ventricular assist device. However, such devices have drawbacks which limit their use to applications having too brief of a time period to provide a real lasting benefit to the patient.
An alternative procedure also involves replacement of the heart and includes a transplant of a heart from another human or animal into the patient. Such a technique also has certain drawbacks. For example, the number of potential donor hearts is far less than the number of patients in need of a natural heart transplant. In addition to requiring removal of an existing organ (i.e. the natural heart) from the patient for substitution with another organ (i.e. another natural heart) from another human, the substitute organ must be xe2x80x9cmatchedxe2x80x9d to the recipient, which is difficult, time consuming, and expensive. Furthermore, a risk continues to exist that the recipient""s body will still reject the transplanted organ and attack it as a foreign object.
Other attempts to assist the heart, such as wrapping skeletal muscle tissue around the natural heart for contraction, or using an external bypass system, such as a cardiopulmonary (heart-lung) machine, are also accompanied by certain problems or drawbacks. Enveloping a substantial portion of the natural heart with a pumping device for rhythmic compression has also been an attempt to address heart failure. Such attempts and other solutions are set forth in greater detail in U.S. patent application Ser. No. 09/850,554, filed May 7, 2001, entitled xe2x80x9cHeart Wall Actuation Device for the Natural Heart,xe2x80x9d which is incorporated herein by reference in its entirety.
Another solution, mechanical ventricular wall actuation, has shown promise. As such, devices have been invented for mechanically assisting the pumping function of the heart, and specifically for externally actuating a heart wall, such as a ventricular wall, to assist in such pumping functions.
Specifically, U.S. Pat. No. 5,957,977, issued Sep. 28, 1999, entitled xe2x80x9cActivation Device for the Natural Heart Including Internal and External Support Structures,xe2x80x9d which is incorporated herein by reference in its entirety, discloses an actuation device for the natural heart utilizing internal and external support structures. U.S. patent application Ser. No. 09/850,554, also discloses actuation devices.
For determining the long-term efficacy of such actuation systems, their operation around and on the heart must be determined. While testing on live animals, and possibly humans, might be desirable, testing on hearts studied out of the body, or ex vivo, is often more practical and desirable. It has been known since the late 19th century that a heart may be isolated ex vivo as a model for the study of myocardial function. This was initially done with animal hearts, requiring the use of animal-derived data to extrapolate to the human condition. Later, human hearts were obtained from cardiac transplant recipients and were restored to a beating condition by being placed on a cardio-pulmonary bypass circuit immediately after explantation from the body.
However, existing heart explantation systems have had some limitations and have precluded the study of global functions, including valvular apparatus, as well as the myocardium. For example, one such explanted heart system operates utilizing models which work via balloons placed in one or both of the ventricles. Such balloons totally bypass the valves and rigidly fix the base of the ventricle. In such a system, the volume assessment of the ventricles has been based either on balloon volume or on assumptions, such as conductance or impedance measurements.
Some systems use crystal-based sonomicrometry techniques which are invalid if ventricular shape changes during an intervention. Also, such systems use large numbers of sonocrystals which make the system expensive. Still further, it is time consuming to precisely position and place the large number of sonocrystals.
An additional problem with existing explanted heart systems is that the hearts are generally suspended by their base, which creates an intracavitary gravitational pressure gradient (from base to apex) that, in diastole, approaches or exceeds the physiologic pressure range for the heart.
Accordingly, it is desirable to provide a system for testing an explanted heart which addresses shortcomings of existing systems, and provides useful data regarding the effect of external conditions on the human heart.
More specifically, it is desirable to have a system which is adaptable to either animal hearts or to human hearts removed for transplant, and which allows previously difficult or impossible ex vivo assessments of devices utilized in the treatment of heart failure. Specifically, it is another objective of the invention to provide such an ex vivo system for testing devices which, either statically or dynamically, change or assist the ventricular operation of the heart to address the effects of heart failure.
It is still another objective of the invention to provide such an assessment system for determining the effect of other factors, such as pharmacological compositions, on the human heart within an ex vivo system.
These objectives and other objectives will become more readily apparent from the further description of the invention set forth below.
The present invention provides a system and method for studying a beating explanted heart. To invention is utilized to study the characteristics of the heart, and to provide ex vivo assessments of external devices and pharmacological compositions on the heart, such as in the treatment of heart failure or heart disease. The present invention provides a system which creates accurate pressure-volume curves and reduces gravitational effects on the explanted heart. The system maintains valvular function in a nearnormal state while the heart is being studied to determine the effect of interventions on the operation of the valves. The system further provides a high-fidelity, high-frequency ventricular volume determination without requiring the expense and placement of a large number of individual sensors.
In one embodiment, a fluid inflow circuit, including an inflow chamber and a compliance chamber, is coupled to an explanted heart to direct fluid for flowing into a ventricle of the heart, such as through the atrium, or directly into the ventricle through a ventricular inflow valve. The fluid outflow circuit is configured to direct fluid flowing out of the ventricle of the beating heart. Respective flowmeters are coupled with the inflow circuit and the outflow circuit for measuring the inflow rate of fluid directed into the heart ventricle, and out of the heart ventricle. A pressure sensor is coupled to the ventricle being tested, which may be the right ventricle, left ventricle, or both ventricles, for measuring ventricular pressure in the ventricle. A processing system receives the inputs from the respective flowmeters and the pressure sensor for graphing pressure changes vs. volume changes and generating curves which are indicative of the performance of the heart. The flowmeters provide varying flow rates over time which are converted into incremental volume changes over time in the volume of the heart by the processing system. The incremental volume changes are then added and/or subtracted from a relative volume of the heart ventricle. A baseline volume determination may be made or proximated by echocardiographic images utilizing geometric assumptions or Simpson""s Rule, for example. One suitable inflow circuit comprises an inflow chamber wherein the pressure of the inflow fluid may be selectively varied. In one embodiment, the inflow chamber might be coupled to an overflow device which is vertically adjustable with respect to a level of fluid in the inflow chamber for varying the pressure in the chamber and thereby vary the pressure on the fluid flowing into the heart. Alternatively, or simultaneously, the inflow chamber might be coupled to a compliance chamber which creates a variable pressure in the inflow chamber. A resistor, or throttle valve, is coupled between the inflow chamber and the overflow device for regulating the effect of the overflow device on the fluid pressure of the inflow chamber.
An outflow circuit comprises an outflow chamber for collecting fluid flowing out of the heart ventricle wherein the pressure of the outflow chamber is selectively adjustable for varying the pressure of fluid flowing out of the heart ventricle. The outflow circuit may comprise a compliance chamber coupled to the outflow chamber which creates the variable pressure. One embodiment of an outflow compliance chamber includes a flexible diaphragm positioned in the outflow chamber for interfacing with the fluid in the outflow chamber and preventing a direct air-fluid interface in the outflow chamber, such as to prevent frothing of blood pumped through the system. Pressure sensors coupled in the inflow and outflow chambers provide measurements of the pressures therein.
A collection apparatus is positioned with respect to the explanted heart for capturing fluid which may leak from the beating explanted heart. The collection apparatus is coupled in line with the inflow circuit to deliver the captured fluid to the inflow chamber for ultimate delivery into a ventricle of the heart. To that end, a heart/lung machine might be coupled between the collection apparatus for oxygenating and treating the fluid, such as blood, which is flowing in the system, and delivering it to the inflow chamber. The heart/lung machine will include appropriate pumps for delivering fluid to the inflow chamber. A dialysis circuit might also be coupled for further treating the blood or other fluid pumping through the explanted heart. Oxygenated blood is also provided to coronary vessels of the explanted heart by a suitable sub-circuit.
Fluid received by the outflow chamber might be directed back into the collection apparatus for being pumped back into the inflow chamber, such as through the heart/lung machine. Alternatively, the output of the outflow chamber might be directed back into the inflow chamber. The outlet tube or line from the outflow chamber includes a resistor or throttle valve for adjusting the resistance to flow out of the outflow chamber.
In accordance with one embodiment of the present invention, the collection apparatus, which is configured for capturing fluid which may leak from the beating explanted heart, comprises a flexible bag or sack which is configured to surround the heart and collect leaking fluid. The bag or sack is suspended or floated within a bath of fluid, which surrounds the bag and provides an outer pressure on the bag and the explanted heart therein, to counteract any fluid pressure inside of the heart. In that way, gravitational effects of fluid within the suspended, explanted heart, are reduced. The flexible bag includes an outlet for draining fluid which may include a rigid tube to prevent blockage of the outlet and enhance the draining of fluid, such as into a heart/lung machine to be delivered back to the inflow chamber. Alternatively, a rigid collection apparatus, such as a sink device.
The processing system of the invention receives inputs from the various sensors and meters, such as the flowmeters, ventricular pressure sensor, and the fluid pressure sensors in the inflow and outflow chambers, and processes the data to provide desirable pressure-volume curves. Other inputs from the heart, such as ECG/EKG inputs and average flow rates, might also be utilized by the processing system.
In one embodiment of the invention, the system is configured for testing one side of the heart, such as the left or right side of the heart. However, the inventive system is also applicable to testing the entire heart, and essentially the components for one side of the heart are duplicated significantly for the other side of the heart, although some components are commonly used for both sides of the heart.